1. Field of the Invention
The present invention relates to a technique for measuring the radiation power of a small wireless terminal and more particularly, to a technique for simplifying the internal mechanism of a coupler to prevent an increase in the size of an apparatus, achieving accurate coupling, and accurately measuring the total radiated power of the wireless terminal even when there is a loss in a system, in a method and apparatus that measures the total radiated power of the wireless terminal using the coupler which includes a space that forms a spheriod and is surrounded by a metal wall surface and in which radio waves emitted from the wireless terminal that is arranged at one focal position are concentrated on a receiving antenna that is arranged at the other focal position.
2. Description of Related Art
With the advent of a ubiquitous society, the explosive growth of subminiature wireless terminals, such as wireless apparatuses related to RFID (wireless tag), UWB (Ultra Wide Band), and BAN (Body Area Network), is expected.
In general, these wireless apparatuses do not have a test terminal due to restrictions in dimensions or for economic reasons, like the wireless apparatus according to the related art. Therefore, it is necessary to receive radio waves emitted from the apparatus in order to test the apparatus.
In particular, the radiation power of the small wireless terminal is strictly regulated considering, for example, an influence on other communication and an influence on the human body, and the measurement of radiation power is an important test item.
Examples of the radiation power include EIRP (equivalent isotropically radiated power) in an arbitrary direction and total radiated power (TRP) emitted to the entire space. The structure of an apparatus for measuring EIRP is complicated and it takes a longtime to measure EIRP. Therefore, TRP is generally treated.
The following TRP measuring methods have been known.
(1) A spherical scanning method that scans a spherical surface including a device under test with a probe, measures radiation power at a mesh point, and adds the measured radiation power.
(2) A method that rotates and mix radio waves emitted from a device under test with a metal blade to generate a random field in a metal covered chamber and estimates the total radiated power of the device under test on the basis of a statistical method.
(3) A method using a pyramid-shaped space covered with a metal film and a device called a G-TEM cell which generates TEM waves in a radio wave absorber.
(4) An electromagnetic wave coupling device that includes a plurality of antennas, isolators connected to the antennas, a phase regulator, and a synthesizer which synthesizes the signals of the array antennas and measures the radiation power of an object to be measured which is disposed on the center line of the array.
The spherical scanning method can measure radiation power with high accuracy, but has problems in that it requires large equipment (for example, a radio anechoic chamber and a spherical scanner) and it takes a long time to measure radiation power.
In addition, the spherical scanning method receives the radio waves emitted to a very small portion of the entire space, calculates radiation power, and adds the radiation power. Therefore, reception sensitivity at each measurement point is very low and it is difficult to measure low level spurious signals.
The method that agitates radio waves in the metal covered chamber does not require a large radio anechoic chamber, but has problems in that there is ambiguity in the consistency between the random field that is artificially generated and a theoretical stochastic model, there is large uncertainty since the method is based on a statistical process, and it takes a long time to measure the radiation power. In addition, it is difficult to measure low level spurious signals, similarly to the spherical scanning method.
In the G-TEM cell, it is difficult to ensure the uniformity of an internal electric field distribution and a biaxial rotating table needs to be provided in the G-TEM cell such that the direction of an object to be measured is changed to all directions, in order to measure the total radiated power.
As a technique for solving the above-mentioned problems, the inventors proposed a method of measuring the total radiated power of wireless terminal using a coupler having a spheroidal closed space (International Publication No. WO/2009/041513).
In the measuring method, an object to be measured and a receiving antenna are arranged at the focal positions of the closed space which forms a spheroid obtained by rotating an ellipse about an axis linking the focuses and is surrounded by a metal wall surface, radio waves emitted from the object to be measured are reflected from the wall surface and are then concentrated on the receiving antenna, and the total radiated power of the object to be measured is measured.